1. Field of the Invention
This invention relates to a stabilized halogen-containing polymer composition, preferably a poly(vinyl halide) composition; more particularly, this invention relates to a poly(vinyl chloride) (PVC) composition comprising a stabilizer and at least one metal salt of a carboxylic acid having at least two carboxyl groups, or an aromatic carboxylic acid having at least one carboxyl group, particularly disodium adipate (DSA).
2. Description of Related Art
Many halogen containing polymers and copolymers are thermoplastic in nature, and, therefore, require heating in order to soften them during fabrication operations such as molding, calendering, and extruding. The heat with which these polymers are processed is usually in the range of 160.degree. C. to 250.degree. C., and more typically 180.degree. C. to 225.degree. C. and is limited in degree and duration by their tendency to deteriorate and decompose. Decomposition of the polymers leads to deterioration of the resultant physical properties of the resin. In addition, a severe darkening in color of the resin results which prohibits its use in lightly colored or transparent articles. This deterioration and decomposition is a major drawback to the use of poly(vinyl chloride) resins in many applications, and in order to overcome this drawback several products have been proposed as stabilizers for halogen containing vinyl polymers. In general, the stabilizers fall into three main classes, metal soaps, organic chemicals, and organometallic compounds.
Vinyl halide polymer compositions typically contain one or more metal-containing heat stabilizers. Metal-containing stabilizers include compounds where the metal is lithium, sodium, potassium, magnesium, calcium, zinc, strontium, cadmium, barium, manganese, iron, cobalt, titanium, aluminum, tin, lead, bismuth or antimony, or a mixture of any two or more such metal-containing compounds. The use of many of these metal-containing compounds as PVC stabilizers is described in British Patent Specification 1,001,344 and an article by Robert G. Weiler, "Uncommon Metals as Heat Stabilizers for Flexible Vinyl", pages 534-537, Volume 28, Technical Papers, Society of Plastics Engineers, Inc., 1982.
Organotin compounds, particularly those having tin bonded to sulfur, such as those which contain a mercapto group, have been found to be an extremely effective type of stabilizer for halogen containing polymers, such as poly(vinyl chloride) polymers. A continuing goal is to develop compositions with equal or better heat stability properties than comparable compositions using organotin stabilizers which are relatively expensive. It is desirable to replace at least a portion of the tin present in a given resin formulation with a less expensive material. It is also desirable to enhance the stabilizing properties of less efficient organotin compounds by addition of inexpensive synergistic additive compounds. Improved stabilized compositions could result in obtaining longer processing times at elevated temperatures and to be able to process resin formulations at higher temperatures.
It has been characteristic of stabilizing systems that they may not fully prevent the deterioration of resins or polymers, including poly(vinyl chloride), during the period when the resins and stabilizer compositions are heated during processing. During this period, which may be from 5 to 30 minutes or longer, the various ingredients including e.g., pigment, plasticizer, stabilizer, lubricant, etc., may be mixed with the resin and the mixture subjected to the influence of heat and pressure to form a substantially homogeneous mixture. Under these processing conditions (T=180.degree. C. or greater), the resin deteriorates, thereby shortening the period that the resin mixture can be heated.
In injection molding processes, heated resin compositions are injected into molds. In general, the viscosity of the resin composition drops as temperature increases. Cycle times of injection molding processes could be reduced by increasing the temperature of the resin composition. It would be useful to be able to raise the processing temperatures of injection molding and other processes.
Well-known metal-containing heat stabilizers for vinyl halide polymers include compounds of the elements of Groups IIA and IIB of the Periodic Table of the Elements, as well as compounds of tin, lead, bismuth and antimony. Conventional well-known heat stabilizers for vinyl chloride polymers are also discussed in Chapter 9 of The Encyclopedia of PVC, edited by L. I. Nass (M. Dekker, New York 1976).